Phosphorothionate insecticides must be metabolized by cytochrome P-450-dependent monooxygenases into their oxons to be potent anticholoinesterases. Because the oxon can be readily detoxified, and because it can inhibit a variety of non-target esterases as well as bind non-specifically to proteins, it is rather unlikely that much of the hepatically-formed oxon could be available to inhibit brain acetylcholinesterase (AChE) to cause toxicity. The project will test the following hypothesis: Because of the efficient protective mechanisms in the liver and blood which prevent hepatically-generated oxon from reaching the target AChE in the brain, the monooxygenase-mediated activation of phosphorothionate insecticides in the brain contributes significantly to the development of acute symptomology. The project will investigate how important the phosphorothionate activiation in the brain is to the poisoning phenomenon by 1) characterizing this activation activity and by 2) quantitating the possible protective mechanisms existing in liver and blood which could provent hepatically-generated oxon from reaching target AChE in the brain. The project will study: phosphoroothionate activation and detoxication, and other monoocxygenase activites in the brain, compared to similar activity profiles from the liver; oxon hydrolysis; in vitro sensitivity of target AChE and protective liver and blood esterases to oxon inhibition, and the recovery of esterase activity; lifetime of active anticholinesterase in the blood; non-specific oxon binding to and release from proteins; and correlation of lethality with brain AChE inhibition. Metabolism will be studied radiometrically, or spectrophotometrically by assay o the anticholoinesterase activity of the oxon and the absorbance of the leaving group. Oxon concentration will be assessed by the quantitation of anticholinesterase activity; esterases will be monitored spectrophotometrically. Surgical techniques will isolate brain from liver to monitor the significance of each metabolism. The project is expected to define the importance of each metabolic activity and protective capacity in the overall toxic response of an organism to a phosphorothionate. The resultant information, by contributing to a better understanding of the factors influencing acute toxicity, will allow better predictions of the therapy required for a victim of an accidental poisoning by a phosphorothionate insecticide.